1. Field of the Invention
The present invention relates to a cathode ray structure for a cathode ray tube, and more particularly, to a cathode structure for a cathode ray tube capable of maximizing thermal efficiency and minimizing power consumption.
2. Background of the Related Art
FIG. 1 is a diagram showing the structure of an already-known color cathode ray tube.
In general, the color cathode ray tube is composed of the following: a front glass panel 3; a funnel 4 including a fluorescent screen 1 inside surface of the funnel 4, the fluorescent screen 1 being covered with R, G and B fluorescent substances or phosphors; a shadow mask 2 with a color selection function; and a tube-shaped neck portion 4a welded to a rear side of the funnel 4.
Housed inside the neck portion 4a of the funnel 4 is an electron gun 5, and outside is a deflection yoke for deflecting electron beams emitted from the electron gun 5 in the horizontal and vertical directions.
Further, R, G, and B electron beams 7 emitted from the electron gun 5 are focused and accelerated by respective electrodes included in the electron gun 5, horizontally and vertically deflected by the deflection yoke 6, and eventually landed at a designated position on the fluorescent screen 1, displaying a desired image.
In other words, the electron beams 7 emitted from the electron gun are properly deflected in the horizontal and vertical directions by the deflection yoke 6. The deflected electron beams 7 then pass through beam passing holes on the shadow mask 2, and strike the fluorescent screen 1 at the front, thereby displaying a designated color image.
FIG. 2 is a diagram showing the structure of a related art electron gun in the color cathode ray tube.
The related art electron gun is largely composed of a tripolar (or triode) unit and a main lens unit. The tripolar unit with a built-in heater consists of a cathode 11, a control electrode 12, and an accelerating electrode 13 for controlling and accelerating thermoelectrons that are radiated from the electrode 11. The main lens unit includes a focus electrode 14 for focusing and eventually accelerating the electron beams generated at the tripolar unit, an anode 15 that is a final accelerating electrode, and a shield cup 16 mounted on the anode 15.
Here, the control electrode 12 is earthed, a voltage ranging 500–1000V is applied to the accelerating electrode 13, and a high voltage ranging 25–35 kV is applied to the anode 15. Applied to the focus electrode 14 are an intermediate voltage, e.g. 20–30% of the applied voltage to the anode, and a dynamic focus voltage.
In this kind of color cathode ray tube, R, G and B electron beams emitted from the electron gun 5 are deflected by the deflection yoke 6 and landed at the fluorescent screen 1, thereby displaying a designated image.
As mentioned earlier, the cathode ray tube usually includes the electron gun 5 for emitting electron beams. Further, there is a bead glass for fixing the cathode structure for radiating electron beams and the electrode structure for focusing and accelerating the electron beams.
FIG. 3 is a diagram explaining a cathode structure of the electron gun.
Referring to FIG. 3, the cathode structure is composed of the following: a base metal 42 to which an emitter 41, covered with an electron-emissive material, is applied; a sleeve 43 having a built-in heater, wherein the upper end of the sleeve 43 is inserted into the base metal 42; and a holder 45 for holding or fixing the lower end of the sleeve 43 inside the cathode ray tube through a strap 44.
The emitter 41, which is applied to the base metal 42, is covered with an electron-emissive material. The electron-emissive material has barium (Ba) as an active ingredient and further includes an alkaline-earth metal carbonate containing strontium (Sr) and calcium (Ca).
The sleeve 43 has a cylindrical shape and a built-in heater. Its front end is inserted into the base metal 42. The sleeve 43 is fitted in the cylindrical-shaped holder 45, which has a larger diameter than the sleeve 43. In short, the upper end portion of the sleeve 43 is enveloped by the base metal 42, and the lower end portion of the sleeve 43 is encompassed by the holder 45, while the central portion of the sleeve 43 is exposed to the outside. The lower end portion of the sleeve 43 and the upper end portion of the holder 45 are welded to each other through the strap 44, whereby the sleeve 43 is safely fixed inside the cylindrical-shaped holder 45.
To apply this related art cathode structure to the cathode ray tube, it is first installed in the electron gun 5 having the structure shown in FIG. 2, and the electron gun 5 is inserted into the cathode ray tube, as illustrated in FIG. 1. Then the cathode ray tube is heated up at about 600° C. and undergoes an exhaustion process to evacuate inside the tube.
Following the exhaustion process, the cathode goes through an activation process in which the cathode is heated at a high temperature in the range of 900–1000° C. Through these sequential processes, the alkaline-earth metal carbonate composing the emitter 41 becomes semiconductive and emits electrons when a certain voltage is applied to the electron gun 5.
Whether intended or not, the cathode is inevitably put in a high temperature environment, both during the manufacturing process and in the operation of the cathode.
The above operations subject the cathode structure and the electron gun including the same to a high temperature environment wherein they are heavily influenced by heat. How much the electron gun can be free of unnecessary influences of heat determines the performance and quality of the electron gun, and furthers picture quality and quality in general of the cathode ray tube mounted with the electron gun.
From this perspective, it is important to know where within the cathode structure the heat flows. First, the heater inside the sleeve 43 radiates heat, and this radiant heat is transferred to the sleeve 43. Most of the transferred heat to the sleeve 43 is conducted to the base metal by heat conduction. The heat is further conducted from the base metal 42 to the emitter 41 and oxidizes the alkaline-earth metal carbonate composing the emitter. In particular, an alkaline-earth metallic oxide is reacted with a very small amount of magnesium, silicon, or tungsten contained in the base metal 42, and shows semiconductive property. In this way, when a designated voltage is applied to the electron gun 5, the cathode emits electrons.
Some of heat in the sleeve 43 is also conducted to the strap 44 and subsequently transferred to the holder 45.
There are multiple routes of heat transfer in the related art cathode structure: one route in which the heat radiated from the heater is transferred to the base metal 42 via the sleeve 43; and the other route in which the heat radiated from the heater is transferred to the holder 45 via the strap 44 that connects the sleeve 43 and the holder 45. The route of heat transfer, or heat transfer mechanism, is totally dependent on positions of each component of the cathode structure, and often gives rise to problems like deterioration of thermal efficiency and increase in power consumption that are be described below.
Total height of the related art structure, for example, is much larger than the sum of the height of the base metal 42 and the length of the holder 45. Also, the length of the sleeve 43 is larger than twice of the length of the strap 44.
The above structural features lead to the following problems for the cathode structure embodying the same.
As described before, the radiant heat from the heater is transferred to the sleeve 43 by heat radiation. Most of the heat transferred to the sleeve 43 is subsequently conducted to the base metal 42 and the strap 44. The heat transferred to the base metal 42 is further conducted to the emitter 41.
In summary, the radiant heat from the sleeve 43 is transferred to other parts via two different mechanisms: one part of the radiant heat transferred to the sleeve 43 is conducted to the base metal 42 and the strap 44 by heat conduction, and the other part of the radiant heat is lost to outside by heat radiation.
To elaborate on the heat lost by radiation from the sleeve 43, the portion of the sleeve 43 that is not covered or encompassed by the base metal 42 and the holder 45 is exposed to the outside, making it an effective radiator through which much of the radiant heat is lost to outside. Meanwhile, the portion being covered or encompassed by the base metal 42 and the holder 45 does not lose much radiant heat. In other words, since the total height of the related art cathode structure is greater than the sum of the height of the base metal 42 and the length of the holder 45, part of the sleeve 43 is inevitably exposed to the outside. This structural limit consequently gives rise to a thermal efficiency problem through which heat is lost to the outside.
Another drawback or problem with the related art cathode structure is that the heat from the sleeve 43 is transferred by heat conduction not only to the base metal 42 but also to the strap 44. That is, when less heat is conducted to the strap 44, more heat is conducted to the base metal 42. Considering that it is better for the base metal to have more heat, the related art cathode structure should be able to reduce the heat being conducted to the strap 44. However, there is a limit in the capacity of the related art cathode structure for reducing the amount of heat that is conducted to the strap 44.
In general, the calories, Q, being transferred by heat conduction can be calculated by the formula, Q=k×A×(T1−T2)/1×t, in which ‘k’ denotes a heat conductivity referring to an amount of heat energy being transferred through a material (medium), ‘A’ denotes a cross-sectional area, ‘T’ denotes a temperature, ‘t’ denotes a time, and ‘l’ denotes a length. In short, the calories conducted, Q, are in inverse proportion to the length of the material.
In the related art cathode ray tube already described, the length of the strap 44 is 50% smaller than the length of the sleeve 43. Because the strap 44 is shorter than the sleeve 43, much heat is lost from the sleeve 43 to the strap 44, causing deterioration of thermal efficiency. When the thermal efficiency is lowered, the heater consumes more power. As such, it gets more difficult to attain low power consumption.
Overall, unless the base metal 42, the sleeve 43, and the holder 45 composing the cathode structure are relocated at their optimal positions (height, space, length etc.), and unless the length of the strap 44 and the height of a welding point of the strap 44 are optimally conditioned, the related art cathode structure will continuously have the same problems, e.g. low thermal efficiency, high power consumption, and deterioration of performances of other components thereby.